TL;DR - We're changing the encoding of Events in Zap to more closely match the ActivityStreams specification. There may be minor compatibility issues with events between Zap and some other platforms on the short term. Currently this affects seamless federation of events and event RSVPs between Zap, Friendica, Hubzilla, and possibly the "hometown fork" of Mastodon.
Detail: In some earlier ActivityPub implementations receipt of a Create/Event activity *implied* an invitation to that activity and RSVP responses could be sent directly to the Create/Event activity. According to the ActivityStreams vocabulary specification, this isn't the correct way to do it; although this mechanism is being used by several platforms today. Going forward we will not send Create/Event at all, but only Invite/Event; although we will also accept events and RSVPs from platforms that still use the "implied Invite".
The work on Zap is complete and will be pushed to the dev branch momentarily. I'm currently working on a patch for Hubzilla over Zot6 and will also attempt to patch Hubzilla's ActivityPub Protocol plugin in the next several days. Whether or not Friendica changes this behaviour on their end is up to that project's developers. The hometown fork of Mastodon also supports implied events and the developer of that fork is aware of this work but I'm not certain of the current development status. So there is likely to be a period of time until the dust settles where seamless event and RSVP federation between these projects may be a bit unpredictable.